1. Field of the Invention
The present invention relates to a process for separating air by cryogenic distillation and to an installation for implementing this process.
2. Related Art
In general, the objective of an engineer creating a process for separating air is to minimize the expenditure of energy.
It is well known to use, for producing oxygen with low energy, a double air separation column which is applied, in particular, on the one hand, so as to minimize the delivery pressure of the air compressor, by reducing the head losses in the exchange line and reducing the temperature difference at the main vaporizer, and, on the other hand, to maximize the oxygen extraction efficiency, by reducing the temperature difference in the exchange line, by choosing a high number of theoretical distillation trays and by installing a sufficient number of sections of structured packings or trays.
Thus, low-pressure columns have four sections of structured packings or trays, including two sections between the bottom of the low-pressure column and an intake for rich liquid, this being an oxygen-enriched liquid taken from the bottom of the medium-pressure column. These two sections are necessary for providing high-performance distillation in the bottom of the low-pressure column. Thus, the medium-pressure columns have four sections of structured packings or trays, including two sections between the liquid air intake and the point of withdrawal of lean liquid.
The purified and compressed air sent to the columns cools in an exchange line comprising which would normally have a volume of more than 200 m3, and therefore with a ratio of the total air volume sent to the exchange line to the volume of the exchange line that would be approximately 2000 Nm3/h/m3 in the case of the example described below.
The refrigeration required for the distillation is frequently provided by an air stream sent to a blowing turbine that feeds the low-pressure column and/or an air stream sent to a Claude turbine. The ratio of the quantity of air sent to the exchange line to the volume sent to the blowing turbine would normally be between 5/1 and 15/1 in the case of the example described below.
In certain cases when energy is not expensive, or even free, it is profitable to reduce expenditure on equipment, while increasing energy requirements.
It is an object of the present invention to reduce the investment cost of an air separation installation and to increase its energy by reducing the size of the exchangers (and therefore increasing the head losses and the temperature differences in the exchange line, and increasing the temperature difference at the main vaporizer), by reducing the size of the distillation columns (by minimizing the number of theoretical trays and the number of sections of packings or trays) and by reducing the size of the refrigerating turbine (by increasing its intake temperature in order to reduce its output).